Our goal is to understand the role Na,K-ATPase plays in the heart and our studies are directed toward several specific areas. These include 1) identifying the role of phosphorylation of the alpha1 subunit by protein kinase A in the regulation of cardiac function, in particular contractility and heart rate, 2) determining whether the cardiac glycoside receptor site of the Na,K-ATPase serves a physiological function, and 3) identifying and characterizing the gene responsible for a new insertional mutant that has been identified in our laboratory which affects cardiac function. This mutant does not directly involve Na,K-ATPase but originated out of our transgenic studies of Na,K-ATPase gene expression. It has recently been shown that the alpha1 subunit of the Na,K-ATPase is phosphorylated by protein kinase A and Na,K-ATPase was previously shown to be regulated by hormones and ligands which use protein kinase A as the terminal regulator in their signal transduction pathways. Thus, it is our goal to determine whether the protein kinase A phosphorylation of Na,K- ATPase plays a role in regulating heart function. Using ES cell and gene targeting techniques, the mouse phosphorylation site (serine 943) will be replaced with an alanine and the control of heart function including contractility and heart rate will be studied in the phosphorylation minus animals. While it is known that Na,K-ATPase is the receptor for cardiac glycosides, a class of drugs used in the treatment of congestive heart failure and certain arrhythmias, it is not known whether the binding site is of physiological importance ina the absence of cardiac glycosides, i.e., is there an endogenous regulator of the Na,K-ATPase which acts at the digitalis site. Using ES cell-gene targeting techniques, our approach is to produce mice with an altered mouse alpha2 isoform of the Na,K-ATPase such that it is fully functional but no longer responsive to cardiac glycosides. This subunit is present in the heart and is normally sensitive to cardiac glycosides. If the animals are normal even under conditions known to increase the plasma levels of the circulating inhibitor, it would be concluded that this site is either not used in vivo or if so, it does not play a significant physiological role. On the other hand, an altered phenotype either during development or in heart function demonstrates that the cardiac glycoside binding site of the Na,K-ATPase plays a physiological role and that an endogenous ligand must exist. These studies are linked via a common goal, namely to describe the physiological role of Na,K-ATPase in cardiac function. Finally, an insertional mutant has been isolated as part of our gene regulation studies which alters cardiac function and the associated gene responsible will be identified.